1. Field of the Invention
The present invention relates to techniques of data transfer between two computers via a network, and in particular to a system and method of transferring streaming data from one computer to another and a repeater for use in such a data transfer system.
2. Description of the Related Art
There has been a streaming technique of distributing a real-time content such as audio or video, allowing a client computer to play the content almost concurrent with receipt of the content from a server computer. Since the client computer can play the content without downloading all the content data, a user can obtain fast response and it is effective especially in the case of small memory capacity in the client side.
In real-time content transmission in Internet, the following protocol may be employed: RTSP (Real-time streaming protocol, RFC 2326) and RTP (Real-time transport protocol RFC 1889). RTSP is a streaming control protocol allowing play, stop, and setup. RTP is a transfer protocol for real-time packet transfer. As application software for streaming transmission, RealSystem™ of RealNetworks and Windows Media Player™ of Microsoft Corporation are available.
In the case of a network such as Internet that does not sufficiently assure assignment of bandwidth or stability of delay, the quality of streaming data transmission is significantly affected by variations in delay time of packet. Since delays in packet delivery vary during transmission, it is necessary to adjust the content play timing at the client side even if the server has sent packets of the content at the same timing as played at the client side.
In order to compensate for momentary delays in packet delivery caused by delay variations to achieve uninterrupted sound or motion, the client side buffers received streaming packets before it starts playing. Such a buffering method is in general employed (see Japanese Patent Application Unexamined Publication No. 4-188929).
However, in the case of Internet access via mobile environment, a radio link causes a wider range of delay variation in packet delivery compared with fixed environment because the radio link itself exhibits a relatively long delay as well as loss of packet. Accordingly, there are cases where the above conventional delay variation control at the client side cannot compensate for delay variations in packet delivery. Since an amount of delay variation that can be compensated for is dependent on the capacity of a buffer, a wider range of delay variation cannot be compensated for without a larger capacity of a buffer. In mobile environment from which a high performance as in fixed environment cannot be expected, it is difficult to ensure a sufficient capacity of a buffer in a mobile device.
It is also considered that delay variation in packet delivery is controlled at some midpoint of a route from the server to the client to improve the quality of streaming data. However, since Internet traffic includes not only streaming data but ordinary data not requiring real-time transmission, it is necessary to discriminate between streaming data and ordinary data, resulting in complicated packet control.